In an elliptical headlight, also referred to as a headlight with optical imaging, the occulting or masking screen constitutes a diaphragm, one edge of which defines the cut-off profile which is reproduced to infinity in front of the vehicle, by the objective, which in this case consists of the lens, whereby to form an illuminating beam which includes a cut-off in a form which corresponds to the cut-off profile.
The diaphragm reproduced, or imaged, by the objective enables an obscurity/clarity limit to be obtained, the form of this limit being fully defined according to requirements, with a high degree of clarity or a desired amount of fuzziness.
Recent legislation has made it necessary to apply improvements to elliptical headlights of conventional types, such that they are able to form specific illuminating beams which are adapted to prevailing travelling conditions, for example either a regulation lighting beam for use in wet weather, which is referred to as “adverse weather lighting” (AWL) or a “rain beam”, or a regulation lighting beam for town driving, or else a regulation lighting beam for motorway (thruway) driving, and so on. The regulation lighting beam for use in wet weather has the particular feature that it includes a cut-off profile identical to that of a regulation dipped or passing beam, and it includes a zone of reduced illumination which is situated below the cut-off line and within the lighting beam.
In this connection, for this type of beam the legislation calls for a reduction in the amount of illumination below the cut-off profile and to the left (in the case where the vehicle is to be driven on the right hand side of the road) of the longitudinal axis of the vehicle.
The zone of reduced illumination is intended to prevent drivers travelling in the opposite direction being dazzled by light reflected off the left hand side of the road between the two vehicles driving in opposite directions, such reflections being due to the road being wet.
Elliptical headlights have previously been proposed for forming a regulation lighting beam of this kind for use in wet weather.
In the document FR-A-2 831 497, it was proposed to arrange a transverse tongue in front of the focal plane of the lens, in such a way that it would be de-focussed with respect to the lens, and so that it forms in the lighting beam a zone of reduced illumination with a relatively fuzzy contour.
This solution is not entirely satisfactory, because the tongue completely masks part of the light rays, so that the zone of reduced illumination has the appearance of a dark patch within the lighting beam. This dark patch may be worrying to the driver of the vehicle in which the headlight is mounted.
In addition, it is difficult to control the influence of the tongue on the light rays of the lighting beam, and this can give rise to unwanted variations in the distribution of light within the beam.
Moreover, the elliptical headlight is generally arranged to produce a plurality of regulation light beams, in particular a conventional dipped or passing beam, since the lighting beam for wet weather corresponds to only one particular travelling situation.
To this end, the elliptical headlight is provided with at least one occulting or masking screen which is movable to a plurality of positions corresponding to a plurality of regulation lighting beams, one of which is the wet weather or rain beam.
In the document FR-A-2 831 497, a masking screen is arranged to tilt about a horizontal transverse axis between an occulting position and a retracted position. This screen includes a cut-off profile corresponding to a regulation passing beam, and also a transverse tongue which is designed to define the zone of reduced illumination corresponding to the rain beam.
The tongue is fixed on the front face of the screen, and it extends vertically upwards. In consequence, it forms in the lighting beam a zone of reduced lighting which extends as far as the cut-off line.
The lighting beam obtained is not entirely satisfactory because the tongue adversely affects the quality of the lighting beam close to the cut-off line.
Another disadvantage of the unfocussed tongue is that it creates an imbalance on the moving screen. The presence of this imbalance is particularly detrimental in a headlight which includes a drum member mounted for rotation about an axis which is inclined in the horizontal plane, such as the conical drum which is described in French published patent document FR-A-2 815 310.
In this connection, the out of balance effect produced by the unfocussed tongue on the pivoting member may be a source of vibrations within the headlight, and it can give rise to premature wear of the motor that drives the drum.
In consequence, this solution is not fully satisfactory, and may give rise to problems of operational reliability of the headlight.
In addition, the presence of the unfocussed tongue may give rise to size problems within the headlight, and in particular mechanical interferences, as a function of the various angular positions occupied by the occulting screen.